VERTEBRATES 7 



6. Circulation of the Blood (see p. 15). The capillaries of the lungs 

 unite into larger and larger tubes, the pulmonary veins, which convey 

 the bright-red blood to the heart. (In the fishes only, which see, is the 

 blood conducted directly from the gills to the body.)' The heart is a 

 hollow muscle, divided into separate compartments, which drives the 

 blood in rhythmic beats or pulsations, after the manner of a pump (the 

 beat of the heart), through the arteries all over the body. The rhythmic 

 beats of the heart are rendered manifest in these vessels, as the pulse. 

 The large arteries divide up into finer and finer branches, and finally, 

 as in the lungs, terminate in capillaries, which penetrate and enmesh 

 all the parts of the body. 



Through the walls of the capillaries an exchange of gases is effected 

 in a reverse direction to that in the lungs. The, oxygen penetrates into 

 the parts of the body (muscles, bones, nerves, etc.), wliilst carbonic acid gas 

 passes from these parts into the blood. 



The oxygen gas so taken up now unites chemically with the solid 

 particles of the muscles, nerves, etc., and inasmuch as every chemical 

 union of a body with oxygen is termed combustion, the muscles, nerves, 

 etc., may be said to be slowly, but steadily, burning away. Just as 

 carbonic acid gas is always produced in the combustion of carbonaceous 

 bodies (e.g., a, candle), this poisonous gas, which has to be removed from 

 the body, is also formed in the combustion (i.e., respiration) of animals. 

 Since, moreover, every combustion is attended by the production of heat 

 (this need not always assume the form of flame, as may be seen in the 

 process of putrefaction e.g., in manure-heaps), so also combustion in 

 animals is accompanied by the development of heat, which we term body 

 heat. (The more energetically we move about, as in bodily labour, 

 running, etc., the deeper is our breathing, the more oxygen is conveyed 

 to the blood, and the more heat is generated in our body.) 



The blood, which has now become richer in carbonic acid gas, and 

 has at the same time assumed a dark-red colour, collects in bloodvessels 

 of larger and larger size, the veins, which conduct it to the heart. 



From the heart it is pumped into the lungs, where, as already 

 explained, it is freed from carbonic acid gas. In this manner the blood 

 constantly pursues its way through the body, the whole process consti- 

 tuting what is termed the circulation of the blood. 



The blood of vertebrata consists of a colourless fluid, in which are 

 suspended an innumerable multitude of very small, disc-like red bodies 

 (corpuscles), which impart to it its red colour. 



In order to put a steam-engine into action, it is necessary to light a 

 fire beneath the boiler, which gradually consumes the coal or wood. In 

 exactly the same manner, as we have seen, particle after particle is con- 



